Unipolar sensing abnormalities: incidence and clinical significance of skeletal muscle interference and undersensing in 228 patients.

Compared to bipolar lead configurations, unipolar pacing systems presumably enhance sensing of cardiac electrical events but are ore susceptible to electrical interference, including skeletal myopotentials. The incidence and clinical significance of oversensing and of undersensing by unipolar R-wave inhibited pacemakers in 228 patients were assessed by 24-hour Holter monitoring and/or by pectoral muscle exercises. Overall, 38% of patients exhibited oversensing and false inhibition due to skeletal myopotentials. Symptoms due to oversensing occurred in 14% of patients and 58% of these required corrective intervention. The presence of silastic coating on the pulse generator had no effect on the sensing of myopotentials. In addition, the incidence of undersensing as assessed by Holter monitoring was 17% despite adequate implantation R wave amplitudes. Thus, oversensing remains a major clinical problem when using unipolar pacemaker systems and their use has not eliminated undersensing.